The Third Generation Partnership Project (3GPP) has begun on work on the development and design of the next generation mobile communications system (a.k.a., as the 5G mobile communication system or simply “5G” for short). 5G will encompass an evolution of today's 4G networks and the addition of a new, globally standardized radio access technology known as “New Radio” (NR).
The large variety of requirements for NR implies that frequency bands at many different carrier frequencies will be needed. For example, low bands will be needed to achieve sufficient coverage and higher bands (e.g. mmW, such as near and above 30 GHz) will be needed to reach the required capacity. At high frequencies the propagation properties are more challenging and high order beamforming at the base station (e.g., eNB or gNB) will be required to reach sufficient link budget. For example, narrow beam transmission and reception schemes may be needed at higher frequencies to compensate the high propagation loss. For a given communication link, a beam can be applied at the transmission point, TRP, (i.e., a transmit (TX) beam) and a beam can be applied at the user equipment (UE) (i.e., a receive (RX) beam)), which collectively is referred to as a “beam pair link” (BPL) or just “link” for short.
Beamforming implies transmitting the same signal from multiple antenna elements of an antenna array with an amplitude and/or phase shift applied to the signal for each antenna elements. These amplitude/phase shifts are commonly denoted as the antenna weights and the collection of the antenna weights for each of the antennas is a precoding vector. Such antenna weights and precoding vectors are examples of a transmit spatial filtering configuration.
Different transmit spatial filtering configurations (e.g., different precoding vectors) give rise to a beamforming of the transmitted signal and the weights can be controlled so that the signals are coherently combining in a certain angle direction as seen from the antenna array in which case it is said that a transmit (TX) beam is formed in that direction. Hence, in some contexts, when we refer to a TX beam we are referring to a particular transmit spatial filtering configuration (a.k.a., “beamforming weights” or “beam parameters”), and when we refer to an RX beam we are referring to a particular receive spatial filtering configuration. If the antennas of the array are placed in two dimensions, i.e. in a plane, then the beam can be steered in both azimuth and elevation directions with respect to the plane perpendicular to the antenna array.
Beamforming generally requires some form of beam management, such as beam search, beam refinement, and/or beam tracking, to determine what UL and/or DL transmit (TX) and receive (RX) beams to use for communication between two units. Typically, the two units are 1) an access network node (ANN) (a.k.a., transmission and reception point (TRP)), such as, for example, a 5G base station (gNB) or other base station, and 2) a user equipment (UE) (i.e., a stationary or mobile wireless communication device (WCD), such as, for example, a smartphone, a tablet, a sensor, a smart appliance (or other Internet-of-Things (IoT) device), etc., that is capable of wireless communication with a TRP).
A beam management procedure refers to discovering and maintaining a beam pair link. An example of the beam management procedure is shown in FIG. 1. In FIG. 1, there is shown a BPL between a TRP 105 (e.g., a base station) and a UE 110 (e.g., wireless device). The BPL comprises a transmit (TX) beam 115 and a corresponding receiving (RX) beam 120. In some embodiments, the BPL may be established and monitored by using measurements on downlink reference signals used for beam management. In New Radio (NR), 3GPP has agreed to use channel state information reference signals (CSI-RS) as the reference signals for beam management. The CSI-RS for beam management may be transmitted by the TRP 105 periodically, semi-persistently or aperiodically (event triggered). Such CSI-RS may be shared between multiple UEs or the CSI-RS may be UE-specific.
As shown in FIG. 1, in order to find a suitable downlink (DL) TX beam, e.g., the TX beam 115, the TRP transmits CSI-RS in different TX beams 115, 125, 130 on which the UE 110 performs reference signal receive power (RSRP) measurements and reports back a number, N, of the best TX beams. The number N of the best TX beams may be configured by the network. The TRP 105 may determine a suitable TX beam (e.g., the TX beam 115 in the example shown in FIG. 1) for the UE 110 based on the reports. This process of the TRP 105 determining the suitable TX beam may be referred to as a TX beam management procedure or as a P2 sweep. The TRP 105 can then repeatedly transmit the CSI-RS on the determined TX beam 115 to allow the UE 110 to evaluate different RX beams to find a suitable RX beam (e.g. RX beam 120). The UE 110 may evaluate the different RX beams by performing CSI-RS measurements of the CSI-RS transmissions. This evaluation may be referred to as a P3 sweep (a.k.a. RX beam training procedure). The suitable RX beam that the UE 110 chooses as a result of the P3 sweep will be agnostic to the TRP 105 in NR. Hence, there is no need for the UE 110 to signal back to the TRP 105 which RX beam it chooses. However, in order to quickly transition from beam management to data transmission, 3GPP has agreed that the UE 110 can be configured to report CSI in relation to the P3 sweep. The channel state information (CSI) report may contain precoding matrix indicator (PMI), rank, and modulation and coding scheme (MCS) for the CSI-RS resource corresponding to the UE-selected RX beam. Referring back to FIG. 1, the UE 110 may transmit, to the TRP 105, a CSI report containing the PMI, rank, and MCS for the CSI-RS resource corresponding to the RX beam 120.
There are basically three different implementations of beamforming, both at the TRP and at the UE: (1) analog beamforming, (2) digital beamforming, and (3) hybrid beamforming. Digital beamforming is the most flexible solution, but also the costliest due to the large number of required radios and baseband chains. Analog beamforming is the least flexible, but cheaper to manufacture due to reduced number of radio and baseband chains. Hybrid beamforming is a compromise between the analog and digital beamforming. 3GPP has agreed to study the concept of antenna panels as one example of analog/hybrid beamforming antenna architecture for NR access technology in 5G. An antenna panel is a rectangular antenna array of dual-polarized elements with typically one transmit/receive unit (TXRU) per polarization. An analog distribution network with phase shifters is used to steer the beam of each panel. Multiple panels can be stacked next to each other and digital beamforming can be performed across the panels. FIGS. 2-3 illustrate antenna panels 202A-B, 302A-B according to the exemplary analog/hybrid beamforming antenna architecture. FIG. 2 shows a first panel 202A and a second panel 202B where each panel comprises a two-dimensional antenna array of dual-polarized elements and is connected to one transceiver unit (TXRU) 204A-D per polarization. FIG. 3 shows a first panel 302A and a second panel 302B where each panel comprises a one-dimensional antenna array of dual-polarized elements and is connected to one TXRU 304A-D per polarization.
In LTE, reference signals (RSs) used for channel estimation are equivalently denoted as antenna ports. Hence a UE can estimate the channel from one antenna port by using the associated RS. One could then associate a certain data or control transmission with an antenna port, which is equivalent to say that the UE shall use the RS for that antenna port to estimate the channel used for data transmission.
In NR, 3GPP has agreed that CSI-RS resources used for beam management can consist of one or two CSI-RS ports. If the TRP has a dual-polarized antenna panel (e.g., a panel as illustrated in FIGS. 2-3), one possible working assumption in NR for beam management is to configure each set of CSI-RS resources with two ports, one port per polarization, i.e. each TX beam is transmitted by the TRP over two polarizations. In this case the UE can measure an average RSRP over both polarizations for each port and report the best TX beam(s) back to the TRP.
In NR, it is expected that the UE will use two or more antenna panels, preferably pointing in different directions, in order to improve the coverage and increase the order of spatial multiplexing. One example of such implementation is illustrated in FIG. 4, where two one-dimensional panels 404A-B are located in different directions at a UE 402. The antenna elements of the two panels 404A-B may be either dual-polarized or single-polarized.
In some scenarios, both the TRP and the UE may have at least two dual-polarized antenna panels each. Accordingly, both the TRP and the UE may have at least four baseband chains and up to four-layer transmissions may be possible between them. Four layer transmissions may be common for 5G, even in cases such as line-of-sight scenarios.